casting 101 C
Controlling Hardness in Aluminum Castings STEVE ROBISON, AFS SENIOR TECHNICAL DIRECTOR
W
hen purchas- ing aluminum castings, most buyers and engineers will
specify the quality required based on the fi nal component’s end use, typi- cally demanding mechanical proper- ties such as tensile strength, yield and elongation. Occasionally, buyers will ask for a specifi c Brinell hardness in the casting. A recent discussion with an aluminum metalcasting facility brought this issue to light. A customer required an 85
minimum Brinell Hardness number (BHN) for an aluminum component sand cast in A356.0 alloy and heat treated to a T6 temper. T is standard raises several issues for the metalcast- ing facility to consider. Where on the casting will the hardness reading be taken? Will an aluminum casting have consistent BHN hardness everywhere or will it vary? What factors would cause variations or lower than neces- sary hardness values? What process controls can reduce variations and ensure the standard is met? Hardness is the resistance of a
material to deformation, particularly permanent deformation, indentation or scratching. Several tests can mea- sure hardness, and each type has its own defined scale of measure. The Brinell test measures resistance to indentation. ASTM E10 describes the Brinell test methods and is the common reference cited for cast metals. The Brinell test is performed by applying a defined load to a small metal ball and measuring the size of the casting surface indentation. The impression diameter is read with a manual scope or optical reader. For most aluminum alloys, a 500kg test load is applied to a 10mm tungsten carbide ball for 10 seconds. (Some harder aluminum alloys may re- quire a bigger load and European standards are slightly different, so metalcasters should check the cast- ing print for the relevant standard.) The hardness values (listed in E10)
are based on the applied load and the diameters of the ball and impres- sion. The test results may be subject to some variation when impres- sion diameters are read manually or hardness impressions are too close together. It is worthwhile to recheck the hardness when a low reading is obtained.
Factors Affecting Hardness
In this case, the customer request for an 85+ BHN in an A356 T6 alumi- num sand casting is not easy to achieve (70-75 is more typical). T e metalcaster may want to discuss the basis of this requirement and the casting’s fi nal ap- plication with the buyer. Here are a few factors that will aff ect hardness: Variability: Metal castings are not
homogenous, and hardness varia- tions will occur throughout the typical casting, though the variation between locations should not be dramatic. Par- ticularly, the cooling rate of the casting aff ects dendrite arm size and will have an eff ect on a hardness test. Casting sections closer to gates and risers may have slower solidifi cation that would lead to larger secondary dendrite arm size and lower BHN than rapidly cooled areas (with smaller DAS). Heat Treatment: Poor heat treat-
ment controls can cause softer cast- ings and increased variability. T e T6 or T61 heat treatment (solution, quench and age) also can be modi- fi ed to improve hardness. T e primary factor during heat treatment is the magnesium level in the alloy and the precipitating hardening process. T e solution heat treat portion of the T6 should redistribute the magnesium and put it back into solution. Factors that could infl uence the uniformity of this redistribution and the eff ectiveness of
the heat treat process are: • Solution temperature: T e higher the better for A356 (1,000-1,005F). Premium heat treat facilities that can maintain tight temperature controls on their furnaces can go to 1,010F without risk of incipient melting. • Solution time: T e foundry should
• Quench rate: T e delay time from so- lution bath to quench bath should be less than 20 seconds. Excessive delays can allow the magnesium to come out of the solution and start precipitation. It is important that the magnesium has been dissolved into the matrix and fi nely dispersed.
check part racking (to ensure adequate heat transfer) and test to see if there are cold and hot spots within in the furnace. It’s impor- tant to confi rm that all castings are reaching full solution temperature and held long enough.
• Quench temperature: In general, the colder the better, unless the part geometry makes it subject to distor- tion. Avoid boiling water. How the castings are put onto the rack also can be an issue. Part confi guration and improper racking can create air pockets that collect steam, which slows the cooling rate.
• Aging temperature and time: Colder and slower aging provides more uniform results. For example, 315F for eight hours might give the same average hardness as 330F for four hours, but there is more potential for variability at higher temperatures. Chemistry: To reach the higher
Brinell measurement (85+) in sand castings, the metalcasting facility would probably need to increase the magnesium to the upper limit of the alloy specifi cation (or higher) for a better heat treat response. However, the additional hardness may reduce ductility to below typical A356 levels. Grain Refi nement & Modifi ca-
tion: Proper grain refi nement (500- 1,000 microns) will help even out magnesium distribution and reduce hardness variability. Casting Quality: Micro shrinkage
porosity can reduce BHN and will in- crease its variability. If there are signifi - cant diff erences in the BHN near risers or in thicker sections than the balance of the part, micro porosity may be a cause. Foundry personnel should check gating and riser layout to ensure adequate feed metal during solidifi cation.
Sept/Oct 2014 | METAL CASTING DESIGN & PURCHASING | 47
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